In the measurement of a solution's conductivity or resistivity by the alternating current bi-pole method, an apparatus generally includes an electrode-supporting body and a pair of electrodes. The electrodes are disposed at certain predefined intervals from each other and from the electrode-supporting body. These distances are defined according to the particular solution's composition and concentration.
At the interface between the electrodes and the sample solution, an electric double layer is produced due to the separation of charges or the distance between the electrodes. Where the sample solution has a different composition or concentration than that for which the electrode distances are defined, a condenser capacitance (polarized capacitance) from the electric double layer produces a large error in the measurement.
To compensate for this error, the following actions have been taken: (a) the voltage applied between the electrodes has been changed; (b) different instruments where the surface area of the electrodes has been altered are used to customize the current density applied; or (c) the measuring frequency has been altered. These actions have not produced a more accurate probe, and merely change the resistivity between the poles or alter the cell constant.
In the prior art measurements of electrolyte solutions, different configurations were provided for similar solutions having different concentrations. Two or these configurations are illustrated in FIGS. 7 and 8.
FIG. 7 illustrates an apparatus for measuring the conductivity of high concentration solutions. An electrode-supporting body 50 has an elongated end which supports two electrodes 52. At the edges of the elongated end 53, covers 51 are provided. This H-shaped configuration provides a certain interval, or path length, L1 for current to travel through the high concentration solution.
FIG. 8 illustrates an apparatus for the measurement of conductivity of low concentration electrolyte solutions. An electrode-supporting structure 60 supports a base 63 which provides two electrodes 62. A sheath or cover 61 flares out from the end of the electrode-supporting structure 60. The path length L2 between the electrodes 62 in FIG. 8 is much less than the path length L1 between the electrodes 52 in FIG. 7. This operates to reduce the cell constant as much as possible in order to minimize both external induction and errors caused by electrostatic capacitance.
Thus, it has been necessary to provide separate devices for measuring the conductivity of solutions having different concentrations or different compositions. Furthermore, it has been necessary to use different conductivity devices having different cell constants, depending upon the concentration and composition of the solution. Where the conductivity of solutions is measured over a wide range of compositions and concentrations, it has been necessary to provide several types of measuring devices, different in form and construction, depending upon the conductivity of the solution.